While we may see more historical cases of Covid-19 in New Zealand return weak-positive tests, Dr Ashley Bloomfield says he's confident they're not infectious.
There's been a handful of historical cases returning weak-positive tests in New Zealand.
RNZ reporter Tom Kitchin got sick with the virus in March and was cleared in early May. In the past few days he tested positive and then negative again.
He told Morning Report he had a very sore throat, felt tired and a bit unwell so felt he had nothing to lose in getting tested again.
"I was expecting it to be negative. It took 72 hours and waiting for that long was pretty dreadful ... when I got the results it was a weak-positive. The DHB rang me and I was in complete shock, I just couldn't believe it really."
As well as two or three cases like Kitchin's, director general of health Ashley Bloomfield says a number of people have returned weak-positive cases after coming through the border.
"It's still a small number but our sense is we may see more of these so we're just looking at first of all how we manage them but also how we report them."
He's confident these people won't spread the virus and says a study was done in South Korea which followed a group of several hundred people who returned a subsequent positive test, were isolated and contact traced. It found no one else was infected by these cases.
People who return a weak-positive test are treated as if they are a positive infectious case until more testing is undertaken, he said.
"The [cycle threshold] number that is the cut off is generally about 30, early in an infection the cycle threshold is lower, it's sort of under 25, around 30 you start to wonder but if it's over 35 you're sort of pretty confident that this is an old infection."
Regarding the case of a person who tested positive after a long incubation period, Bloomfield says officials are still confirming where the infection happened.
The man tested positive for Covid-19 nearly a week after leaving managed isolation may have had an unusually long incubation of the virus.
"It seems most likely that it actually happened on the flight from India through to New Zealand ... because there was a number of cases on that flight and several of them have got the same genomic sequence ... as this person."
The case warrants exploring the options for testing people after they've left managed isolation, Bloomfield said.
This may include testing people who have come from places which have high number of cases, he said.